Tag: Fractions

So how do you convert percentages to fractions and decimals and vice versa? This post will show examples of each.

Convert a percentage to a fraction:

This one is easy as if you remember, a percentage is already a fraction where the numerator is displayed and the denominator is 100. So you just create the fraction and simplify it (see my posts on fractions):

This one is just a matter of moving the decimal point, two places to the left. Keep in mind that the decimal point will not usually show at the end of integer percentage, but you can assume it to be at the end of the number:

37% = 37.% = 0.37

18.5% = 0.185

112% = 1.12

0.15% = 0.0015

Any 0’s at the end of the decimal, can be left off:

40% = 0.40 = 0.4

3. Convert a decimal to a percentage:

This is just the opposite of of the above: you just move the decimal point two places to the right, then add the % symbol:

0.25 = 25% (if an integer results, you can leave the decimal point off)

0.2786 = 27.86%

0.002 = 0.2%

2.345 = 234.5%

4. Convert a fraction to a percentage:

Here you multiply by 100/1, simplify, then multiply numerators together and denominator together. It is advisable to simplify before multiplying:

I am confident that the chance of you liking this post is 100%. But what does 100% mean? I will talk about this in the next few posts.

First let’s review what a fraction is. The fraction

\[
\frac{3}{4}
\]

means that I have 3 pieces (the numerator) of some whole thing that was divided into 4 pieces (the denominator). So the denominator sets the size of the pieces (the bigger the denominator, the smaller the pieces) and the numerator sets how many pieces.

Being creatures that have 10 fingers, we naturally migrate to things that are powers of 10. Way back in the Roman empire days (I remember them fondly), the Romans frequently used fractions that had a denominator of 100. In fact, the word “percentage” comes from the latin (Roman) per centum which means “by the hundred”. That has continued to today, as a percentage is really a fraction where the denominator is fixed at 100. What you see in a percentage is the numerator:

\[
{37}{\%}\hspace{0.33em}{=}\hspace{0.33em}\frac{37}{100}
\]

So a percentage like 37% means that if I divide something into 100 equal pieces, I have 37 of these pieces. Percentages are also used to indicate a probability. My first sentence in this post used a percentage in this way. If you remember, a probability of an event is a fraction. The probability of event A is expressed as P(A):

So 100% of you liking this post means that if I take 100 random people who have read this post, 100 of them will like this post. This means all will like this post as that is what 100% means: the whole:

\[
{2}{.}\hspace{0.33em}\hspace{0.33em}\frac{7}{9}\hspace{0.33em}{+}\hspace{0.33em}\frac{11}{12}
\]. Since these fractions are being added and they have different denominators, we need to find a common denominator. To find the Least Common Denominator (LCD):

9 = 3 × 3, 12 = 2 × 2 × 3

LCD = 2 × 2 × 3 × 3 = 36

So now convert each of the fractions into equivalent ones with 36 as the denominator. So I will multiply the top and bottom of the first fraction by 4 and the other one by 3:

\[
{3}{.}\hspace{0.33em}\hspace{0.33em}{6}\frac{1}{4}\hspace{0.33em}{+}\hspace{0.33em}{2}\frac{7}{8}
\]. First, the fractions need to be converted to improper ones. Then since 4 evenly divides into 8, we just need to convert the first fraction to an equivalent one with 8 as the denominator:

\[
{4}{.}\hspace{0.33em}\hspace{0.33em}\frac{11}{12}\hspace{0.33em}{-}\hspace{0.33em}\frac{7}{9}
\]. These are the same fractions as in the second problem, but now it’s a subtraction problem. We have already converted these two fractions into equivalent fractions with the same denominator, so we will use that result here:

So far I have avoided division of fractions. This is not hard, in fact, it’s almost as easy as multiplying but I would like to show why the method works.

First consider 6 ÷ 2. This problem is asking the question, “How many 2’s fit into 6?”. The answer is 3 sets of 2’s make up 6. If the problem was 6 ÷ 3, the question would be “How many 3’s fit into 6?”. The answer here is there are 2 sets of 3’s that make up 6. Of course, the answer is not always an integer, Sometimes there are leftover numbers. For example, how many 2’s can make up 7, in other words, 7 ÷ 2. The answer is there are 3 sets of 2’s that can fit into 7 but there will be 1 left over as sets of 2’s do not exactly make up 7. Now remember that fractions are indicating division as well so the above problems are equivalent to

So what would \[
{1}\hspace{0.33em}\div\hspace{0.33em}\frac{1}{2}\hspace{0.33em}{=}\hspace{0.33em}\frac{1}{\frac{1}{2}}
\]

mean? It means the same as before: how many one-halves fit into 1? Now the answer is 2 because there are two halves in a whole. What about \[
{2}\hspace{0.33em}\div\hspace{0.33em}\frac{1}{2}\hspace{0.33em}{=}\hspace{0.33em}\frac{1}{\frac{1}{2}}
\]? Can you see that the answer is 4? Because if there are two things split into halves, then there will be 4 halves.

Looks like there are 4 three-quarters that fit into 3, and that is correct. Let’s look at one more. What about \[
\frac{3}{4}\hspace{0.33em}\div\hspace{0.33em}\frac{1}{4}\hspace{0.33em}{=}\hspace{0.33em}\frac{\frac{3}{4}}{\frac{1}{4}}
\]. Can you see that there are 3 one-quarters that fit into three-quarters?

All of the above answers can be obtained by multiplying and remembering that integers like 6 can also be shown as a fraction: \[
\frac{6}{1}
\] as 6 divided by 1 is still 6.

So now let’s look at the previous problems. You can convert a fraction division problem by inverting the fraction in the denominator then multiplying:

So now let’s use the skills from the last two posts to work on mixed fractions. Consider:

\[
{3}\frac{3}{8}\hspace{0.33em}{+}\hspace{0.33em}{6}\frac{7}{8}
\]

As the denominators are the same, one can do this problem by adding the whole parts first to get 9 then add the fractions together to get \[
\frac{10}{8}
\] which after simplifying and converting to a mixed fraction is equal to \[
1\frac{1}{4}
\]. Then you add this to the 9 to get \[
10\frac{1}{4}
\]. But I want to show a more general method that is particularly useful when the denominators are different and/or the problem is a subtraction.

The first step is to convert the mixed fractions to improper ones as discussed in my last post:

The addition in the last step was easy as the denominators are the same. Now all that is left is to simplify and convert to a mixed fraction. You can convert first then simplify which has the advantage of having smaller numbers to factor, so let’s do that:

\[
\frac{82}{8}\hspace{0.33em}{=}\hspace{0.33em}{82}\hspace{0.33em}\div\hspace{0.33em}{8}\hspace{0.33em}{=}\hspace{0.33em}{10}
\] with remainder of 2 so

Now we need to find a common denominator between 8 and 12. To find the least common denominator (LCD), we first factor both numbers:

8 = 2 × 2 × 2, 12 = 2 × 2 × 3

As 2 and 3 are the only factors present, we now combine them, using them only the maximum number of times each appear in the above factorisation:

LCD = 2 × 2 × 2 × 3 = 24

So the common denominator we will use is 24. We want to convert each of the fractions in the problem into equivalent ones that have 24 as the denominator. For the first fraction, we will multiply top and bottom by 3 to get the 24 in the denominator. We will use 2 for the second fraction to get 24 in the denominator there as well:

In my last post, I showed how to convert an improper fraction to a mixed one. Today, let’s do the opposite: convert a mixed fraction to an improper one. Why would one want to do this apart from the pure joy and ecstasy of doing maths? Because, this would be the first step in adding, subtracting, or multiplying mixed fractions together. Notice that I have been avoiding division of fractions. This will be the topic of another post – we are well and truly going to beat the number of Rocky movies!

Let’s first show how to convert a mixed fraction to an improper one then we will use this skill eventually in a problem. Consider

\[
2\frac{3}{4}
\]

Now please realise that this is maths shorthand for \[
{2}\hspace{0.33em}{+}\hspace{0.33em}\frac{3}{4}
\]. The denominator in the fraction part of this is indicating a whole of something divided into 4 pieces. This will be the denominator in our eventual improper fraction as well.

So to convert the 2 whole pieces into the number of total pieces if each whole was divided into 4 pieces is to multiply the 2 by the 4 to get 8 pieces. That is, if you divided 2 whole things into 4 pieces each, you get 8 total pieces.

Now we also have to remember that we have this fractional part of \[
\frac{3}{4}
\]. Remember that this fraction means that we have 3 pieces of a whole that was divided into 4 pieces. So we need to add these 3 pieces to the 8 pieces we have so far to get 11. So we have 11 pieces of some items that were each divided into 4 pieces. In fractional form, this is \[
\frac{11}{4}\]. This is the answer. So to convert a mixed fraction to an improper one, you multiply the denominator part by the whole number part, then add the numerator part. Let’s do another one:

Now the fractions we have been working with are called proper fractions: these are fractions where the numerator is smaller than the denominator. These types of fractions are smaller than one, which is why they are called proper as they are a fractional part of one. This implies there are things called improper fractions, and there are. Not improper in the sense of your drunk uncle at a wedding, but improper because they are greater than or equal to one. These will be fractions with a numerator equal to or greater than the denominator.

For example \[
\frac{5}{3}
\] is an improper fraction. Now they can be added, subtracted, multiplied, and simplified like any other fraction. But when the result of an operation with fractions results in an improper fraction, you are expected to convert this to a mixed fraction – a whole number plus a proper fraction. So \[
\frac{5}{3}
\] is equal to \[
1\frac{2}{3}
\]

So how do you convert an improper fraction to a mixed one? Just take out the whole parts and leave the resulting proper fraction. You do this by dividing: \[
\frac{5}{3}\hspace{0.33em}{=}\hspace{0.33em}{5}\hspace{0.33em}\div\hspace{0.33em}{3}\hspace{0.33em}{=}\hspace{0.33em}{1}
\] plus a reminder of 2. So in this case, the improper fraction is one whole plus two thirds left over or \[
1\frac{2}{3}
\].

Now try \[
\frac{24}{11}
\]:

\[
\frac{24}{11}\hspace{0.33em}{=}\hspace{0.33em}{24}\hspace{0.33em}\div\hspace{0.33em}{11}\hspace{0.33em}{=}\hspace{0.33em}{2}
\] with a remainder of 2. So \[
\frac{24}{11}\hspace{0.33em}{=}\hspace{0.33em}{2}\frac{2}{11}
\].

I’m starting to feel like the Rocky movie series – will I get to part 7?

Here are more examples of adding fractions with different denominators:

\[
\frac{4}{15}\hspace{0.33em}{+}\hspace{0.33em}\frac{3}{5}
\]

So two different denominators. We have to find equivalent fractions that have the same denominator before we can add these. As mentioned in my last post, first check to see if one of the denominators evenly divides into the other. In this case, 5 divides into 15 three times so 15 can be our common denominator. We just have to convert the second fraction. We do this by multiplying top and bottom by 3 because multiplying the 5 by 3 will give us the 15 we want:

So now that we know how to get the least common multiple (LCM) of two numbers, let’s apply this knowledge to add two fractions together that have different denominators.

So let’s look at:

\[
\frac{3}{4}\hspace{0.33em}{+}\hspace{0.33em}\frac{3}{18}
\]

In my last post, we found the LCM of the two numbers 4 and 18 to be 36. Now that these numbers are in the denominator, we can also call 36 the least common denominator (LCD) – two different terms to describe the same thing depending on the country, school district, or era you find yourself in.

From Part 2 of the posts on Fractions, I covered the method to add two fractions together that have different denominators. The trick is to find a common denominator but now we are more wise and can find a least common denominator. So we will use the LCM we found in my last post to solve the problem.

So I want to convert the two fractions into equivalent ones that have a denominator of 36. I do this my multiplying the top and bottom of the fraction by the same number needed to make the denominator 36:

So are we done? That is the answer but textbooks and teachers will always tell you to “simplify” your answer. That means get the numbers as small as possible. The method of getting equivalent fractions above can be done in reverse to get simpler (but equivalent) fractions. If you can identify factors common to the numerator and denominator, these can be cancelled. Notice in our answer that there is a common factor of 3. That is:

Factors are things multiplied together. So numbers can have many different factors. For example, 12 can be seen as 6 × 2, or 3 × 4, or even 2 × 2 × 3. Notice that in the first two sets of factors, the 6 and the 4 can be further broken down to be 2 × 3 and 2 × 2, but the only factors of 2 are 2 × 1 and for 3, 3 × 1. (I am limiting myself to integer factors here because if other types of numbers are allowed, a number has an infinite number of factors).

So numbers like 2 and 3 that only have themselves and 1 as factors are called prime numbers. Other prime numbers are 5, 7, 11, 13, 17, 19, 23. Notice that the only even prime number is 2 as all other even numbers have 2 as a possible factor.

The process of finding the Lowest Common Denominator (LCD) of two or more fractions is done by factoring the denominators into its prime factors. This post is about factoring. I will then use this new skill to find the LCD in my next post.

Now factoring is easier if you know the times tables, commonly up to and including the 12 times tables. When a textbook says to factor a number, it usually means to get the number down to factors of just prime numbers like we did for 12 = 2 × 2 × 3.

Let’s try some:

9 = 3 × 3, 16 = 4 × 4 = 2 × 2 × 2 × 2, 20 = 4 × 5 = 2 × 2 × 5

So generally you just find any two factors of a number then break those factors down further until all that is left are prime numbers. When asked to do this by hand, the numbers are generally small, say less than 144 (12 × 12). Bigger numbers take more work and are usually done by computer programs. By the way, now that you know the factors of 12, you should be able to immediately write down the factors of 144:

144 = 12 × 12 = 2 × 2 × 3 × 2 × 2 × 3

Some more examples:

35 = 5 × 7, 40 = 4 × 10 = 2 × 2 × 2 × 5

100 = 10 × 10 = 2 × 5 × 2 × 5, 60 = 5 × 12 = 5 × 2 × 2 × 3

To find the LCD of two or more denominators, the first step will be to factor each denominator. I will do this in the next post.